In the heavy duty pneumatic radial tire suitable for use in trucks, buses and the like, it is common to reuse as a base tire for recapping when a tread rubber reaches a service limit in wear because it is not desirable to use a new tire only once. Referring to FIG. 16 illustrating a section of a main part of the conventional tire inclusive of a bead portion, there is often observed a case of causing a long and large crack or a separation failure accompanied with the growth of the crack in an end 4te of a turnup portion 4t of a carcass 4 or an outer end 6e of a rubberized steel cord layer (generally called as a wire chafer) 6 as a reinforcing layer for a bead portion 1 among members constituting the bead portion 1. The used tire having such defects is unsuitable as a base tire and it is obliged to give up recapping thereof. When the service conditions are very severe, the aforementioned crack or separation failure is caused even in the new tire on the way of the service life.
The aforementioned long crack or separation failure in an end portion such as end 4te, end 6e or the like results from the concentration of large strain in such end portion. Referring to FIG. 17 showing a section of another conventional tire, in order to eliminate this strain, there is widely used means that one or more organic fiber cord layers, two layers in the illustrated embodiment, for example, nylon cord layer 15 (15-1, 15-2) generally called as a nylon chafer are arranged outside the turnup portion 4t of the carcass 4 alone in case of FIG. 16 or together with the steel cord layer 6 as the reinforcing layer in case of FIG. 18 and adjacent thereto at a height sufficiently covering the above end portion in a radial direction of the tire.
However, the application or addition of the organic fiber cord layer does not develop an effect to an intended extent and reasons thereof are investigated to elucidate the following facts.
That is, as a part of the two organic fiber cord layers 15-1, 15-2 (see FIG. 17, FIG. 18) is perspectively shown in FIG. 19 showing a half lower-side of a tire running under loading at a zone ranging from a leading edge of a contact patch to a trailing edge thereof, cords C.sub.R (upward to the right in the figure) and cords C.sub.L (upward to the left in the figure) of the organic fiber cord layers 15-1, 15-2 are arranged so as to cross with each other between the layers in a bead portion 1 corresponding to a ground contact region of the tire tread rotating under a given air pressure and under loading, so that even in either case of arrangements upward to the left and the right, the cords have to be always subjected to compression deformation at the leading side or the trailing edge of the contact patch.
Although a detail reason of always causing the compression deformation will be described later, the degree of compression deformation becomes more higher when the traction force and braking force are further applied to the tire. When an axial compression force is applied to the cord in the organic fiber cord layer 15, modulus of the organic fiber cord becomes very low to the axial compression, so that the rigidity required for developing a stress mitigating function of the organic fiber cord layer 15 naturally intended to the turnup end 4te of the carcass 4 or the end 6e of the reinforcing layer 6 is largely diminished. According to experiments, a ratio of axial compression modulus to tensile modulus in the organic fiber cord layer embedded in rubber is only about 0.1.
Also, it has been confirmed that cracking failure is caused in an end portion of the organic fiber cord layer 15 toward the outside of the tire. As a result of an investigation on the cause of the cracking failure, it is elucidated to be caused by a large tensile strain applied to rubber in the vicinity of the end 15e of the organic fiber cord layer 15 toward the outside of the tire. That is, air pressure filled in the heavy duty pneumatic radial tire mainly used in the truck and bus is as high as 7.00.about.9.00 kgf/cm.sup.2, for example, at room temperature and is further increased by rising of tire temperature accompanied with the running of the vehicle. As shown in FIG. 18, a large tension T is applied to the carcass 4 by such a higher internal pressure, and the large tension T produces pulling forces a, b of arrow direction in not only the turnup portion 4t of the carcass 4 but also the bead portion reinforcing layer 6 and the organic fiber cord layer 15 and hence the turnup portion 4t, bead portion reinforcing layer 6 and organic fiber cord layer 15 are forcibly displaced in the acting direction of the pulling forces a, b. By such a forced displacement is particularly caused a large tensile strain e in rubber near to the end 15e of the organic fiber cord layer 15. The tensile strain e is further increased by bending deformation of the bead portion 1 under loading shown by a phantom line. Consequently, fatigue crack is created in rubber near to the end 15e of the organic fiber cord layer 15 by repetitive action of strain amplitude of the tensile strain e accompanied with the rotation under loading, which grows to finally cause the separation failure.
In addition, it is strongly demanded to form a lower section profile of radial ply tires for recent trucks and buses, from which it tends to increase low-section tires. In the low-section tire used under heavy load, the deformation of the bead portion 1 is particularly increased and the strain amplitude quantity of the tensile strain e is considerably increased, so that there is highlighted cracking failure or separation failure in the end 15e of the organic fiber cord layer 15 located toward the outside of the tire, which has hardly been observed in the conventional tire. This type of the failure is a recent tendency and an effective improving countermeasure does not exist at the present time.